1. Field of the Invention
This invention relates to a paper feeding apparatus for a printer, and more particularly to a paper feeding apparatus for a printer by which paper can be fed with a high degree of accuracy.
2. Description of the Prior Art
Various printers have conventionally been used wherein a print head is driven in response to a desired printing signal to perform desired printing on paper.
FIG. 5 shows one of such conventional popular printers. Referring to FIG. 5, a platen 2 in the form of a flat plate is disposed at a substantially central location of a frame 1 of the printer such that a printing face thereof extends substantially vertically. A carriage shaft 3 is disposed in parallel to the platen 2 at a location of the frame 1 forwardly below the platen 2. A guide portion 4 in the form of a flange is formed at a front end edge of the frame 1, and a carriage 5 is mounted on the carriage shaft 3 and the guide portion 4 for back and forth movement along the carriage shaft 3 and the guide portion 4. A print head 6 is mounted at an end portion of the carriage 5 such that it is opposed to the platen 2 and is driven to move into and out of contact with the platen 2 by a driving mechanism not shown. A ribbon cassette (not shown) in which an ink ribbon is accommodated is removably mounted on an upper face of the carriage 5 such that the ink ribbon therein is guided between the print head 6 and the platen 2. Further, a take-up bobbin 7 for taking up the ink ribbon of the ribbon cassette and a supply bobbin 8 for supplying the ink ribbon are disposed on the upper face of the carriage 5.
A paper insertion opening 9 is formed rearwardly of the platen 2 for feeding a paper sheet (not shown) to a location forwardly of the platen 2 therethrough, and a pair of paper feed rollers 10 for transporting the paper sheet at a predetermined speed are disposed adjacent the paper insertion opening 9. A pair of contact rollers 11 are disposed for rotation below the paper feed rollers 10 such that they are individually contacted under pressure by the paper feed rollers 10. A paper feed gear 12 is disposed projectingly on one side face of the frame 1 and is mounted coaxially with the paper feed rollers 10. A motor gear 15 of a paper feed motor 14 is connected to the paper feed gear 12 by way of a plurality of transmission gears 13 so that, when the paper feed motor 14 is energized, the paper feed rollers 10 are rotated by way of the motor gear 15, the transmission gears 13 and the paper feed gear 12 so that a paper sheet which has been inserted through the paper insertion opening 9 and is held between the paper feed rollers 10 and the contact rollers 11 is transported by the paper feed rollers 10 and the contact rollers 11.
In the conventional printer described above, a paper sheet is inserted through the paper insertion opening 9 until it is held between the paper feed rollers 10 and the contact rollers 11, and the paper feed rollers 10 are driven to rotate by the paper feed motor 14 so that the paper sheet is transported at a predetermined speed in a direction perpendicular to the direction of movement of the carriage 5. Further, while the print head 6 is held in contact under a predetermined pressing force with the paper sheet, the carriage 5 is moved and the take-up bobbin 7 is rotated to take up the ink ribbon of the ribbon cassette while the print head 6 is driven in response to a desired printing signal to perform desired printing on the paper sheet.
In the conventional printer of the construction described above, however, since the motor gear 15, the transmission gears 13 and the paper feed gear 12 are set to a predetermined gear ratio so that, when a paper sheet is fed, the paper feed rollers 10 can be driven to rotate by an amount equal to a predetermined one line feed (line space) distance in response to rotation of the paper feed motor 14, for example, if the paper feed motor 14 is driven to rotate the motor gear 15 in order to rotate the paper feed rollers 10 by an amount equal to the one line feed distance from a condition shown in FIG. 6A, then the amounts of rotation of the motor gear 15, the transmission gears 13 and the paper feed gear are different from one another as seen from FIG. 6B. Consequently, even if the motor gear 15 is driven to rotate with a high degree of accuracy by the paper feed motor 14, the paper feed gear 12 suffers from a great error in rotation arising from errors in dimension of and errors in meshing engagement between the motor gear 15, the transmission gears 13 and the paper feed gear 12. Accordingly, the conventional printer is disadvantageous in that a high degree of accuracy cannot be assured in feeding of a paper sheet by the paper feed rollers 10.
Therefore, in order to drive the paper feed rollers 10 with a high degree of accuracy, all of the motor gear 15, the transmission gears 13 and the paper feed gear 12 must be produced with a high degree of accuracy. This, however, is actually very difficult, and it is very difficult to assure a high degree of accuracy in feeding of a paper sheet.